Yarn tensioning systems and methods for keeping yarn which is taken from a yarn storage system to a yarn take-off system in a weaving machine under tension are used in all kinds of weaving machines, such as inter alia carpet weaving machines, velvet weaving machines, wire weaving machines and flat weaving machines.
In this case, the yarn storage system is typically a bobbin creel. This bobbin creel may in this case be a typical bobbin creel for a weaving machine or a typical bobbin creel for a tufting machine, which is used atypically with a weaving machine. With a typical bobbin creel for a weaving machine, the longitudinal axis of the bobbins is more or less perpendicular to the path to be followed by the yarns and the yarn is taken off along the direction of the path. In this case, the bobbin rotates continuously.
With a typical bobbin creel for a tufting machine, the yarn is taken off in the direction of the longitudinal axis of the bobbin. The bobbin is virtually at a standstill. When supplying yarns to a tufting machine using such a bobbin creel, the yarn is supplied as tensionless as possible, with this yarn being guided in tubes. In order to use such a creel in a weaving machine, after a short piece of tube, weights are for example used to keep the warp threads under tension.
The yarn take-off system may be, for example, the weaving zone in a weaving machine, or may be an intermediate store of yarns, in which the yarns of a bobbin creel are redistributed according to the further desired use in the weaving machine and/or assembled to form new yarns which are gathered at this intermediate store for further use thereof in the weaving machine.
With such weaving machines, it is therefore important to keep the tension of the yarns as uniform as possible in various locations in order thus to be able to process the yarns as evenly as possible in the yarn take-off system. With yarns which are supplied to the weaving zone, the quality of the woven fabric may deteriorate significantly when tensions of these yarns in the weaving zone deviate. Generally, when the tension becomes excessively high, yarns may become damaged or even break, or when the tension becomes excessively low, yarns may become entangled. Not only the thread properties of the yarn, but also the path to be followed by the yarn from the yarn storage system to the yarn take-off system, affect the tension of the yarn at the location of the yarn take-off system.
Various systems are already known for keeping yarns which are taken from a yarn storage system to a yarn take-off system in a weaving machine under tension. Some of these yarn tensioning systems are known, for example from GB 2 428 921 A, GB 2 442 955 A, GB 2 378 188 A and EP 1 077 276 A1.
The yarn tensioning system described in EP 1 077 276 A1 is a yarn tensioning system according to the preamble of the first claim for keeping a warp thread running from a bobbin to a weaving zone of a weaving machine under tension and, if necessary, drawing it back. In this case, the warp thread is arranged over two friction rods. A first brake roller runs over the warp thread between both friction rods. A second brake roller runs over the warp thread between the second friction rod and a guide grid. Weights are suspended from the brake rollers. The weights and the friction rods ensure a tension of the yarn which is as even as possible.
However, a problem of this known yarn tensioning system is that when the characteristics of the warp thread (thickness, flexibility, etc.) change, additional weights have to be hung from said weights in order to adjust the forces exerted by the weights on the warp thread. This is a cumbersome, time-consuming and labour-intensive process, as it has to be carried out for each individual warp thread which is passed from a bobbin to the weaving zone. With a velvet weaving machine, there are on average between 1,000 and 10,000 bobbins in the bobbin creel per metre of machine width, depending on the quality of the woven fabric, and the number of different colours present in the woven fabric, with a face-to-face weaving machine the number of bobbins per metre of machine width may even be as much as 32,000 for high-quality woven fabrics comprising many colours.